We seek to understand the molecular basis of cell division, cytokinesis, in animal cells. The central spindle, a set of antiparallel bundled microtubules in the anaphase spindle, regulates formation of the actomyosin- based contractile ring and is essential for completion of cytokinesis. We propose that the organization and function of the central spindle is a consequence of the structural organization and biochemical properties of the centralspindlin complex. Centralspindlin is an evolutionary conserved, multimeric complex containing a kinesin-like protein (ZEN-4/MKLP1) and a Rho family GAP (CYK-4/MgcRacGAP) that is highly concentrated on the central spindle and midbody during anaphase and telophase, respectively. We propose to combine in vitro biochemistry and in vivo rescue assays in C. elegans embryos and in mammalian cells to address three specific aims: (1) To dissect the molecular organization of the centralspindlin complex and to characterize the atypical kinesin protein, ZEN-4. By characterizing the critical protein domains in the centralspindlin complex in relative isolation, we will develop the biochemical framework necessary to understand the function of these molecules in more complex reactions as well as in vivo. (2) To decipher the molecular mechanism of central spindle assembly. Central spindle assembly will be reconstituted in vitro from purified components in order to define the principles by which this set of dynamic components assemble into a highly ordered and stable structure. (3) To determine how the central spindle mediates completion of cytokinesis. Centralspindlin contains two distinct protein domains that are proposed to control late steps in cytokinesis. Therefore, the molecular function of these domains will be established. This research will provide molecular insights into a cellular structure that is critical for cell multiplication. Therefore our research could contribute to the development of novel anti-mitotic agents for the treatment of cancer. Furthermore, molecular dissection of ZEN-4 will enhance our understanding of the mechanism of action of microtubule motors.